Like spawn but allows one to specify explicitly the channel from which
an actor should take its input. Useful for extending the library to work
over other channels.

Building an actor computation

An actor computation can be halted immediately by calling yield,
a synonym for mzero. When an Action calling yield is composed with
another using | the second takes over processing the same input
which the former yield-ed on.

Here is an example of a computation using guard which returns mzero if
the test is false: